1. Field of the Invention
This invention relates in general to structural members and methods of forming structural members. More particularly, but not by way of limitation, it relates to composite, pre-stressed structural members and methods and apparatus for forming, designing and pre-stressing such structural members.
2. Description of the Prior Art
In the prior art there are a wide variety of structural members, both prefabricated and fabricated in place. These structural members include single element members such as steel beams and composite element members such as concrete reinforced with or supported by metal bars or support beams and elements. The combinations and shapes of these types of structural elements are, of course, numerous.
In forming structural members which include concrete elements or which are entirely made of concrete it has often been found desirable to prestress the concrete to reduce tension loads on the concrete. It is well known that concrete can withstand relatively highly compression stresses but only relatively low tension stresses. Accordingly, wherever concrete is to be placed in tension it has been found desirable to prestress the concrete structural member with a compression stress which remains in the structural member and must be overcome before a failing tension will be achieved.
Conventional pre-stressing as performed in the past involves stretching a metal wire or cable through a mold and placing this cable in tension during hardening of concrete which has been poured into the mold. When the concrete has hardened the tension-loaded cable is cut placing a compression load on the hardened concrete. The compression force from the severed cable remains with the element once it is removed from the mold.
A problem with conventional prestressing is that it requires careful calculations to avoid overstressing the cables because it is usually desirable to stretch the cables to near failure to achieve a sufficient pre-stressing. The apparatus, to achieve this pre-stressing, is also complex. Still further, cutting the cables can be a dangerous procedure and can ruin the pre-stressed structural member if not peformed correctly.
In forming structural members for spanning between two supports it has often been found desirable to utilize a steel structural support beneath a molded concrete surface. Because steel can withstand a much higher tensile stress these composite structural members are formed with the steel sustaining most of the tensile stress which is placed on the member. In general, these types of structural members do not include any type of prestressing.
To form composite members of the type having an upper concrete surface and a metal structural support underneath a multiple piece form mold is utilized. First, the steel supports such as wide flange beams are placed beneath a mold assembly having two or more mold pieces disposed about the beam or beams. Next, the concrete is poured into the mold such that the concrete fills the mold and extends over the beam. When the concrete has hardened the mold pieces are disassembled from around the beam such that the concrete rests on the beam. In most instances, these wide flange beam supported concrete structural members are formed in place. This is usually advantageous so that the concrete surface can better fit into the finished structure. Some types of concrete structural forms, however, are prefabricated.
Despite the utility of the structural members utilized in the past, these members have not been completely satisfactory. Accordingly, it is an object of the present invention to provide an improved composite, pre-stressed structural member.
It is particularly an object of the present invention to provide an improved composite, pre-stressed structural member which is less expensive, lower weight, and/or capable of withstanding larger loads in use.
It is also an object of the present invention to provide an improved method and apparatus for forming composite, pre-stressed structural members of the type described. Particularly, it is an object of provide such a method which is simple, low cost, and results in an improved structural member.